1. Field of the Invention
The present invention relates generally to implantable stimulating devices, and more particularly to the measurement and control of stimulation current in implantable stimulating devices.
2. Related Art
The delivery of electrical stimulation has become an established part of medical therapy. Electronic devices are implanted within the body in order to stimulate nerve tissue (e.g. cochlear implants) for perceptual or functional purposes. Such electronic devices commonly use platinum electrodes as the interface between the electronics and the body tissue. In general terms, such electrodes are selectively driven with a current in order to evoke a perception (for example sound) or a function (for example a limb movement) in the user. FIG. 8 provides a schematic illustration of a system for application of electrical stimulation. As illustrated, a plurality of platinum electrodes 4 are connected to an implant 2 via insulated wires 5 and driven by a stimulating current 6 from a current source 3. The stimulating current passes 7 through the tissue 1 and the nerve cell 8, and returns to the implant 2.
It is usual for the stimulating current to be structured as biphasic pulses, in such a way that there is no net charge delivered to the tissue. If, however, the stimulating current is allowed to flow in one direction for too long, toxic products can escape the interface and damage or destroy the surrounding tissue.
The use of charge-neutral pulses (i.e., pulses with a net charge of 0) ensures, in principle, that the requirement that net charge delivered to the tissue is zero is met. In practice, however there will be a small error in the generated stimulation current. Further, if the stimulation current source goes out of compliance, then significant charge errors can occur.
Existing cochlear implants do not readily permit the measurement of stimulation current during stimulation. Presently, voltage telemetry measurements are recorded for a stimulation electrode in vivo as an indication of the existence of the stimulation current. It is assumed that the stimulation current has the amplitude (value) that the current source has been set to generate.
In practice, the value of the stimulation current can be different that the value to which the current source has been set. For example, if the current source is out of compliance, (e.g. when the electrode impedance is higher than normal) then the amplitude of the stimulation current may be much lower than the value to which the current source is set. Or, if there is a current leakage at the stimulation electrode circuitry, (e.g. a fault condition) then the leakage current adds to the stimulation current, resulting in an increase of the current flow between the stimulation electrodes. This increased current flow can vary from very small to very large values depending on the degree of leakage. A user reporting a hearing sensation and/or pain at low stimulation current levels may indicate that the stimulation current is not effectively controlled by the current source.